The connecting together of individual battery cells for high-voltage, high-energy applications is well known. However, the chemical reaction that occurs internal to a battery during charging and discharging typically limits deep-cycle battery life to hundreds of charge/discharge cycles. This characteristic means that the battery pack has to be replaced at a high cost one or more times during the life of a hybrid-electric or all-electric vehicle.
Batteries are somewhat power-limited because the chemical reaction therein limits the rate at which batteries can accept energy during charging and supply energy during discharging. In a hybrid-electric vehicle application, the battery power limitation manifests itself as an internal series resistance that restricts the drive system efficiency in capturing braking energy through regeneration and supplying power for acceleration.
Ultracapacitors are attractive because they can be connected together, similar to batteries, for high-voltage applications; have an extended life of hundreds of thousands of charge/discharge cycles; and have a low equivalent internal series resistance that allows an ultracapacitor pack to accept and supply much higher power than similar battery packs. Although ultracapacitor packs may be more expensive than battery packs for the same applications and currently cannot store as much energy as battery packs, ultracapacitor packs are projected to last the life of the vehicle and offer better fuel-efficient operation through braking regeneration energy capture and supplying of vehicle acceleration power. Furthermore, the price of an ultracapacitor pack has the potential to decrease significantly because of economies of scale in known manufacturing techniques.
During charging and discharging operation of the ultracapacitors, parasitic effects, as modeled by the equivalent series resistance, cause the cell temperature to increase. Cooling is required to minimize increased temperature operation that would degrade the energy storage and useful life of each ultracapacitor.
Other than operation/performance, the key consideration for ultracapacitor packs in a heavy duty hybrid-electric vehicle is heat dissipation. The ultracapacitor cells used in ultracapacitor packs are constructed as layered sheets of conductive material and dielectric, wrapped around a central axis and forming a cylinder. Terminals are placed on each end of the cell. The terminals are typically threaded and provide both an electrical coupling point and a support point. The thermal characteristics of this construction are such that most of heat generated by the cell is transferred to the environment via the two ends of the cell. Currently, heat dissipation is accomplished by blowing cooling air across the cylindrical bodies/cases of the cells.
Ultracapacitor packs in vehicles, especially heavy-duty vehicles, reside in a harsh operating environment and face unique challenges not present in non-vehicular applications. In particular, the vehicular environment is dirty, hot, and subject to vibration. Current implementations attempt to address these problems, but leave room for improvement and innovation.
In heavy-duty transit bus applications and other heavy duty vehicle applications higher performance and smaller size ultracapacitor packs are required, especially where ultracapacitor packs are required to be placed on the roof of the heavy-duty transit bus or other heavy duty vehicle.